(a) Field of the Invention
The present invention relates to signal transmitting/receiving apparatuses. More particularly, the present invention relates to signal transmitting/receiving apparatuses in an orthogonal frequency-division multiplexing (OFDM) system for minimizing a sampling clock error caused by a slight period difference.
(b) Description of the Related Art
In a wireless channel environment, subcarrier power varies and subcarrier phases are distorted. A system for performing orthogonal frequency-division multiplexing (OFDM) to compensate such a distorted signal compensates a channel after performing a discrete Fourier transform.
As a prior art, a paper entitled “An Analysis of Two Dimensional Pilot Symbol Assisted Modulation for OFDM” has been disclosed in International Conference on Personal Wireless Communications (ICPWC) 1997, pp. 71-74.
This prior art relates to a two-dimensional channel compensation method used in the OFDM system. That is, a pilot that satisfies pilot providing interval conditions of a time domain given as Equation 1 and a frequency domain given as Equation 2 is provided to a transmission symbol of a transmitting apparatus, a channel is estimated by using an interpolation method in a receiving apparatus, and data are demodulated through the estimated channel.
                              S          t                <                  1                      B            d                                              [                  Equation          ⁢                                          ⁢          1                ]            
Here, St denotes a time domain pilot providing interval, and Bd denotes Doppler spread.
                              S          f                <                  1                      t            max                                              [                  Equation          ⁢                                          ⁢          2                ]            
Here, Sf denotes a frequency domain pilot providing interval, and tmax denotes maximum delay spread.
In the above prior art, there is a problem in that the pilot providing interval may not be optimized when there is a time difference between the transmitting apparatus and the receiving apparatus. In addition, an OFDM symbol including a first pilot signal is delayed for a predetermined time to obtain a channel estimate value by using the OFDM symbol including the first pilot signal and the OFDM symbol including a second pilot signal in the receiving apparatus.
Accordingly, a system for reporting a transmitted packet decoding error after performing a decoding operation may not appropriately operate due to the OFDM symbol delay.
In addition, as another prior art relating to a radio channel compensation method, a paper entitled “OFDM Physical Layer Specification for the 5 GHz Band” has been disclosed in IEEE P802.11 Wireless LANs, pp1-27, (July 1998).
In this prior art, to prevent a delay between symbols in a channel estimating process, the transmitting apparatus provides a preamble signal to subcarriers of two subsequent OFDM symbols to transmit the OFDM symbols, and provides a pilot signal to predetermined subcarrier sections of other OFDM symbols. The receiving apparatus uses the two OFDM symbols including the preamble signal to estimate a channel, and the other OFDM symbols are modulated based on the estimated channel.
Here, performance may be optimized when a channel variance caused by the two subsequent OFDM symbols is maintained until a last symbol is transmitted. However, in an actual system, since the channel variance may not be maintained, it is difficult to optimize the performance.
FIG. 1 shows a block diagram representing a data packet used in a conventional OFDM system.
As shown in FIG. 1, the data packet includes a cyclic prefix 11 and a data area 12, and it is susceptible to a delay profile in a wireless channel environment.
Refereeing to FIG. 1, the OFDM system estimates a timing error and a frequency error in a time domain and offset the errors when obtaining synchronization by using the cyclic prefix 11.
However, the estimated error causes a remaining frequency offset. In addition, when a time error occurs between the transmitting apparatus and the receiving apparatus, the remaining frequency offset causes an accumulated phase difference between the first subcarrier and the last subcarrier. The accumulated phase difference becomes greater in second and third OFDM symbols than in a first OFDM symbol when it is not compensated in each OFDM symbol, and therefore, it may not be demodulated. Particularly, the accumulated phase difference may problematically cause a signal to noise ratio (SNR) loss.
The accumulated phase difference caused in the conventional OFDM system will be described in Equation 3 and Equation 4.
A sampling clock signal error caused by a sampling period difference between the transmitting apparatus and the receiving apparatus is given as Equation 3.
                              t          Δ                =                                            T              ′                        -            T                    T                                    [                  Equation          ⁢                                          ⁢          3                ]            
Here, T and T′ denote sampling periods of the transmitting apparatus and the receiving apparatus. An effect on a subcarrier Rn,k of an nth symbol received after performing a discrete Fourier transform is given as Equation 4.
                              R                      n            ,            k                          =                                            ⅇ                              j                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                k                ⁢                                                                  ⁢                                  t                  Δ                                ⁢                n                ⁢                                                                  ⁢                                                      T                    s                                                        T                    u                                                                        ⁢                          x                                                                                ⁢                                  n                  ,                  k                                                      ⁢                                                  ⁢            sin            ⁢                                                  ⁢                          c              ⁡                              (                                  π                  ⁢                                                                          ⁢                  k                  ⁢                                                                          ⁢                                      t                    Δ                                                  )                                      ⁢                          H                                                                                ⁢                                  n                  ,                  k                                                              +                      w                                                                      ⁢                              n                ,                k                                                                        [                  Equation          ⁢                                          ⁢          4                ]            
Here, Ts and Tu respectively denote an entire OFDM symbol period and an actually applied data period, and wn,k denotes a additive white Gaussian noise (AWGN).
  ⅇ      j    ⁢                  ⁢    2    ⁢                  ⁢    π    ⁢                  ⁢    k    ⁢                  ⁢          t      Δ        ⁢    n    ⁢                  ⁢                  T        s                    T        u            denotes a phase grating rate according to each subcarrier. When a period difference of tΔ occurs, a phase grating rate difference between the first subcarrier and the last subcarrier may become greater. Particularly, as the number of subcarriers increases, the phase grating rate becomes greater.
In cyclic prefix compensation, an error at a time domain is more precisely estimated than in a packet obtaining synchronization to offset the accumulated phase difference. In this case, compared to a system including a lesser number of subcarriers, more phases remain in a system having a greater number of subcarriers and a plurality of OFDM symbol periods after performing the cyclic prefix compensation. That is, the sampling clock signal error caused by the sampling period difference between the transmitting apparatus and the receiving apparatus may not offset the frequency offset of the accumulated subcarrier by estimating the cyclic prefix by using the cyclic prefix.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.